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German Lorenz cipher machine, used in World War II to encrypt very-high-level general staff messages

Cryptography (or cryptology ; from [Greek] , kryptos , "hidden, secret"; and , gráphō , "I write", or , [-logia] , respectively) is the practice and study of hiding [information] . Modern cryptography intersects the disciplines of [mathematics] , [computer science] , and [engineering] . Applications of cryptography include [ATM cards] , [computer passwords] , and [electronic commerce] .

Until modern times cryptography referred almost exclusively to [encryption] , which is the process of converting ordinary information ( [plaintext] ) into unintelligible gibberish (i.e., [ciphertext] ). Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A [cipher] (or cypher ) is a pair of [algorithm] s that create the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and in each instance by a [key] . This is a secret parameter (ideally known only to the communicants) for a specific message exchange context. Keys are important, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks.

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